The use of a Sieverts’ type apparatus for the evaluation of hydrogen sorption of large amount of material would be impractical since it would require huge reference volumes. For instance, a reference volume of 650 l would be required to measure the hydrogen sorption of 100 g of sodium alanate material, expecting a pressure change of 0.1 bar during the measurement. Therefore, in order to study cycling, kinetics and heat transfer in hydride tanks up to kg scale, a hydrogen tank station was designed and constructed. Basically, the installation allows the quantitative evaluation of the hydrogen charging (discharging) process into (from) a tank, which is filled with a metal hydride (e.g. sodium alanate). The charge (discharge) of gas into (from) the tank and the hydride is determined and controlled by measuring the temperature, pressure and the gas mass flow. A simplified process flow diagram of the installation is depicted in Fig. 2.3. In the appendix the detailed piping and instrumentation diagram (P&ID) of the installation is presented. The accessories and instruments of the installation are rated to work up to 130 bar.
The amount of hydrogen that has been absorbed by the material between two different times t0and tf is calculated from the hydrogen mass balance on the gaseous phase of the tank:
The first and third terms on the right side of the equation corresponds to the hydrogen mass present in the gaseous phase at times t0and tf, respectively. The second term on the right side accounts the total Figure 2.2: Thermocouples positions in the 15 mm diameter cell (thermocell) for heat flow analysis during absorptions and desorptions.
hydrogen that flowed into the system between times t0and tf. Gas density,
, is calculated by means of the van der Waals equation at the temperature and pressure of the tank. At high levels of pressure (>50 bar), the density calculated using ideal gas law has noticeable relative error when compared to experimental density values [10].The volume of the gaseous phase within the hydride tank, Vg, is required for the calculations. Prior to the experiments with hydrogen, this volume was calibrated using Argon, chemically inert to the hydride material. The calibration is based on the following equation (derived from Eq. 2.2 for the case of argon):
Tanks of the company Swagelok of stainless steel 316L were utilized for the scale-up measurements [33]. A schematic diagram of the tank is presented in Fig. 2.4. All handling, including the tank filling procedure with the material, was done inside a glove box with purified argon atmosphere. During measurements, the hydrogen is distributed inside the tank by a sintered metal filter. The filter was equipped with a thermocouple located in its centre, which also corresponds to the centre of the hydride bed. A second thermocouple enters into the hydride from the other end of the tank. Its measuring tip is located in a non-defined position in the bed of material.
On one hand, during the hydrogen absorption heat is released and should be removed to maintain the required driving force for the absorption. During hydrogen desorption, on the other hand, heat should be supplied. In the charging station an external oil circuit regulates this heat management. The hydride
Mass Flow meter
Figure 2.3: Simplified process diagram of the hydrogen tank station.
tank is placed inside a plastic shell made of PEEK (polyether ether ketone), a polymer which has good mechanical properties at temperatures up to 250 °C and a relative low density (approx. 1.3 g ml-1). The set-up of hydride tank and shell is similar to a heat exchanger of “double pipe” type configuration, Fig.
2.5. The outer annular channel is part of the oil circuit. A high-temperature thermostat (T max 400 °C from LAUDA GmbH) pumps the oil in the circuit and also regulates its temperature by means of heating resistances and an additional cooling system. The employed heat transfer oil was Ultra 350 (dibenziltoluene, LAUDA GmbH). This oil was used to minimize the risk associated with a possible leak, since it is non-reactive to sodium alanate material.
(a)
(b)
H2
Stainless Steel Tank
Sintered metal filter Bed of Hydride
TT-3 TT-2
Figure 2.4: Tanks of stainless steel 316L for scale-up measurements. (a): Schematic profile of the tank, sinter metal filter and thermocouples. The filter is equipped with the thermocouple TT-3 located in the centre of the hydride bed. A second thermocouple TT-2 enters into the hydride from the other end of the tank. Its measuring tip is located in a non-defined position in the bed of material. (b): Dimensions of the tanks as presented in the Swagelok catalogue [33]. For the 300 ml tank and pressure rating until 300 bar: T= 6.1 mm, A= 48.2 mm, B= 368 mm, P=¼ inch NPT thread. For the 500 ml tank and pressure rating until 100 bar: T= 2.4 mm, A= 50.8 mm, B= 351 mm, P=¼ inch NPT thread.
The temperature is measured using thermocouples type K from Thermocoax with fast response grounded junction (temperature range of -40 °C to 1300 °C, accuracy of 0.5 °C). The temperature is measured in the hydrogen pipeline, in two positions inside the hydride tank (see Fig. 2.4) and in three positions in the side of the heat transfer oil close to the tank (see Fig. 2.5). The absolute pressure is measured with a piezoresistive absolute pressure transmitter PAA-23 (pressure range of 0 bar to 200 bar, accuracy of 0.01 bar, KELLER AG). The gas flow is measured and controlled with two thermal flow meters (flow range from 0 ln min-1 to 10 ln min-1 and 0 ln min-1 to 250 ln min-1, accuracy of 0.01 ln min-1 and 0.1 ln min-1, respectively. Bronkhorst Mättig). In the thermal flow meters, a voltage signal is produced when a gas is flowing through. This signal is proportional to the mass flow and the heat capacity of the gas. The unit “ln”, although not extendedly used but common in gas flow meters controllers, represents one normal litre e.g. one litre of gas under normal conditions of
Figure 2.5: Hydride tank inside its shell as heat transfer configuration of “double pipe” type. (a):
Schematic of the heat transfer oil pathway in the annular channel. (b): Intern configuration and accessories.
temperature and pressure (0 °C and 1.01325 bar). For instance, a flow of 1 ln min-1 of hydrogen corresponds to a hydrogen mass flow of 0.08991 g min-1, since H ,0 C ,1.01325bar
2
=0.08991 g l-1.The installation was automatically operated by an own developed LabView computer-based application, which accomplishes the on-line measurement, control, monitoring and regulation of the tank station. Temperature, pressure and mass flows are measured and saved on-line in the computer.
Pneumatic valves were installed and controlled from the application. A snap-shot of the designed interface of the application and a general view of the tank station are presented in the appendix. A more extensive description of the experimental procedures and the installation as well as its risk and safety analysis is presented elsewhere [34].
heat transfer
This chapter starts with the experimental results on the effect of heat transfer on the sorption kinetics of sodium alanate material [35]. Further on, developed empirical kinetic models for the absorption and desorption of the material are presented [36, 37]. The last part of the chapter focuses on scaled-up sorptions of the material. The chapter is organized in four different sections that cover the mentioned investigated themes. Each section begins with the description of the experimental results and concludes with the respective discussion.